This invention relates to pole changing, single phase alternating current rotary electric machines, and more particularly to induction motors having stator windings which are capable of alternatively being connected for two pole and six pole operation.
High efficiency heat pumps require high efficiency compressor motors having multi-speed capabilities. Conventional multi-speed motors employ an extra winding that effectively reduces the operating flux level which is equivalent to voltage reduction in a poly-phase motor. The resulting speed torque curve of the motor is reduced and the motor operates at reduced speed determined by the intersection of the load speed torque characteristic with the motor speed torque characteristic. In order to get significant speed reduction, the motor then operates at a high value of slip which leads to very poor efficiency. Since a compressor motor of a high capacity heat pump may operate at low speed for 75% of the pump's operating hours, the use of a two speed compressor motor based on high slippage at the low speed is unacceptable.
Pole changing alternating current electric motors have been developed to provide multi-speed operation without high slippage values. For example, U.S. Pat. No. 3,233,160 issued Feb. 1, 1966 to Rawcliffe, discloses a single phase alternating current pole changing motor having a stator winding arrangement which includes two windings for connection together to a single phase alternating current supply to provide a running field for the motor wherein each winding is wound for a first pole number and has winding parts arranged for alternative connection in the circuit according to a method of pole amplitude modulation, thereby providing second and third pole numbers together in each winding considered independently. An additional stator winding is provided for connection to the single phase alternating current supply through a phase shifting means, to provide a starting field for the motor. The first two windings are physically disposed for elimination of the third pole number from the running field.
U.S. Pat. No. 3,619,730 issued Nov. 9, 1971 to Broadway et al., discloses a four pole/six pole machine which utilizes a pole amplitude modulation technique. The Broadway et al. patent employs four stator field coils in the main winding and favors a four pole field. In addition, Broadway et al. shows only a four pole (consequent pole) auxiliary winding, which would be used only in the four pole connection and would be disconnected when running as a six pole machine. Therefore, the machine could be started only in the four pole mode. The present invention utilizes six field coils in the main stator winding of a two pole/six pole single phase motor, and can be started when connected for either pole number.